Variable compression ratio can significantly increase the fuel efficiency of reciprocating piston internal combustion engines used in passenger cars, light duty trucks and other vehicles. The present invention relates to a variable compression ratio mechanism having an eccentric control shaft for adjusting engine compression ratio.
An engine having an eccentric control shaft is shown by Per Gillbrand in U.S. Pat. Nos. 5,611,301 and 5,562,069. Referring to U.S. Pat. No. 5,562,069, a crankcase (4) is connected to a cylinder head assembly (2) with a hinge shaft (20). Use of the hinge shaft (20) enables the cylinder head assembly (2) to tip relative to the crankcase (4) for adjusting compression ratio. A control shaft (56) is also mounted in a crankcase (4). The engine has only one control shaft (56) per cylinder head assembly (2). The engine includes a straight second shaft (52) mounted in the cylinder head assembly (2), and a plurality of links (50) connecting the control shaft (56) to the second shaft (52). Rotating the control shaft (56) moves link (50) causing second shaft (52) to also move, causing cylinder head assembly (2) to tip relative to crankcase (4) resulting in a change of engine compression ratio. The engine is characterized in that at least one link (50) connects the control shaft (56) to the cylinder head assembly (2). The engine is assembled by sliding hinge shaft (20) into the engine to connect the crankcase (4) and cylinder head assembly (2), and sliding the second shaft (52) into the engine to connect the links (50) and the cylinder head assembly (2). The control shaft (56) includes eccentrics, preventing sliding in of the control shaft (56) to complete assembly. Accordingly, the links include removable bearing caps (60) so that the links (50) may be assembled onto the control shaft (56).
Machining and assembly tolerances are a problem with the variable compression ratio mechanism taught by Gillbrand. In particular, hinge shaft (20), control shaft (56) and second shaft (52) must be parallel for durable operation of the engine. Additionally, the shafts must remain true when the engine is running and exposed to high mechanical loads. Attaining precision alignment of the shafts can be attained, however, an undesirably massive crankcase is needed, and attaining tight machining tolerances is relatively costly. The engine has many links and hinge joints which also adds to manufacturing and alignment costs.
Another problem with the engine is that hinge shaft (20) is located relatively far from crankshaft (6) and the centerline axis of cylinder (10) in order to minimize the degree of tipping required to change compression ratio. Locating hinge shaft (20) and control shaft (56) relatively far from the cylinder centerline axis results in high moment forces in both crankcase (4) and cylinder head assembly (2). The high moment forces further increase the need for an undesirably massive and heavy crankcase. An in-line engine layout is employed to minimize weight and complexity, however the crankcase is still massive.
To accommodate tipping, the engine also includes tall crankcase walls (24) and a flexible gasket (44) between the crankcase (4) and cylinder head assembly (2). Another problem with the engine is noise and vibration because of the high crankcase walls (24) that are not anchored at their top, and the large gasket (44) which does not contain noise within the crankcase.
Another engine having one control shaft per cylinder head assembly is shown by Manousos Pattakos in U.S. Pat. No. 8,166,929. The engine is also characterized in that links (15) connects the control shaft (13) to the cylinder head assembly (9). The control shaft (13) is located generally in line with the cylinder centerline axis in order to minimize moment forces. However, a problem with the engine is that a massive and large cylinder head is needed to accommodate the links and control shaft in the cylinder head. The control shaft is located far above the combustion chamber roof and far away from the top of the cylinder in order to provide room for the links. An in-line engine layout is employed to minimize weight and complexity, however the crankcase is still massive. The engine has a relatively tall engine height which will make packaging in some automobiles impractical. Another problem with the engine is that there are a large number of eccentric bearings and links, which increases manufacturing and alignment cost.
An engine having two control shafts is shown by Daisuke Akihisa in U.S. Pat. No. 7,047,917. A problem with the variable compression ratio mechanism shown in U.S. Pat. No. 7,047,917 is that precision alignment of the two control shafts is required for durable operation of the engine, and attaining the precision alignment is costly. A second problem with the engine is that there are a large number of eccentric bearings, which increases manufacturing and alignment cost. An in-line engine layout is employed to minimize weight and complexity, however the crankcase is still relatively massive.